1. Field of the Invention
The present invention relates to a process for producing an ethylene polymer having a molecular weight distribution of a middle width.
2. Description of the Related Art
Ethylene polymers are used for different purposes depending on their molecular weight distribution. More specifically, polymers having a wide molecular weight distribution are often used for blow moldings; polymers having a middle molecular weight distribution are often used for fibers, tapes and the like; and polymers having a narrow molecular weight distribution are often used for injection moldings such as bottle caps, buckets and the like.
Hitherto, solid catalyst components comprising a magnesium halide and a titanium halide known as catalysts having high activities generally produce ethylene polymers having a narrow molecular weight distribution. For this reason these catalyst components are suitable for the production of polyolefins for injection moldings such as bottle caps and buckets but are generally unsuitable for the production of the other articles as described above.
Recently, there has been a need for development of a catalyst which will produce polymers having a wide molecular weight distribution for the purpose of extending the applications of the polymers, and many methods in which a variety of transition metal compounds are used or a catalyst component is supported on an inorganic oxide carrier have been invented (e.g., Japanese Patent Publication Nos. 37037/1977, 8588/1978, 006/1980, 45247/1982, 13084/1983 and 58364/1987, etc.).
We have previously proposed a process for preparing a solid catalyst component which has excellent particle form and is particularly suitable for slurry polymerization or vapor phase polymerization by treating a composition corresponding to the component (A-1) of the present invention (described hereinafter in detail) with (1) a silicon halide, (2) a titanium halide, (3) a titanium halide and a hydropolysiloxane, or the like (see, for example, Japanese Patent Laid-Open Publication Nos. 127706/1983, 285203/1986, 285204/1986, 285205/1986, 180612/1982, 5309/1983 and 5311/1983). These catalysts, which may be or may have been useful in their own ways, may further be improved if the molecular weight distribution of the polymer obtained is broaden or if their catalyst activities are increased.
We have proposed catalysts prepared by treating a composition corresponding to the component (A-1) with (1) an aluminum halide, (2) an aluminum halide and a titanium or silicon halide, (3) an organoaluminum compound or a hydropolysiloxane and an aluminum halide, or the like (see, for example, Japanese Patent Laid-Open Publication Nos. 12903/1984 and 43007/1984). However, these catalysts, which have a comparatively high activity and provide polymers having a wide molecular weight distribution, may require further improvement in that mass formation of solid particles during the reaction of the aluminum halide thus causing variation of the catalyst activity or nonuniform formation of the particles will be reduced.
Furthermore, there have been proposed catalysts prepared by treating a composition corresponding to the component (A-1) with an organoaluminum compound and a titanium or silicon halide (e.g., Japanese Patent LaidOpen Publication No. 225104/1983). However, these catalysts often had activities and widths of their molecular weight distribution at unsatisfactory levels, and thus improvements have been desired.
In addition, a catalyst prepared by reacting a reaction product of a magnesium dihalide, a halogenated Si compound and a Ti compound with an alkylaluminum halide has been proposed in Japanese Patent Laid-Open Publication No. 212209/1982. An ethylene polymer produced with this catalyst, as far as we know, had a narrow molecular weight distribution. Thus it was difficult to obtain such a product as an object of the present invention.
An object of the present invention is to provide an ethylene polymer having a molecular weight distribution of a middle width by using a catalyst having a high activity and is intended to be accomplished by the use of a specific catalyst.